Why Neurotics Haven't Died Out

How many children people have can be predicted by their
personality, at least in certain populations with high birth rates, a new study
finds.

For women, high levels of neuroticism, a personality trait
meaning a tendency to feel anxious and moody, was associated with having more
children. Not surprisingly, extroversion was linked to more offspring for men.

The study, conducted in Senegal, is one of only a handful to
address the questions: Why do human personalities differ to begin with, and how
has evolution
shaped these traits? The Senegalese population studied is thought to more
closely resemble the environment in which humans evolved.

It suggests why some seemingly disadvantageous personality
traits, such as neuroticism, may have stuck around over the course of
evolution. And it indicates that there is more to personality than just the
ability to make a good first impression.

"You mustn’t think of personality traits only in terms
of how they affect your happiness, or how other people might perceive
you," said study researcher Markus Jokela, of the University of Helsinki in
Finland. "When you're considering the evolutionary origins of personality,
then obviously reproductive success and behaviors related to reproductive
success are very very important."

Personality and
fertility

Most past studies focused on the evolution of human
personality have been carried out in Westernized countries — populations in
which access
to birth control has allowed couples to have fewer children; and quality
hospital care means fewer die young, both factors that are not characteristic
of pre-industrial societies.

Jokela and his colleagues studied four villages in rural
Senegal that practice polygamy and have higher birth rates and mortality rates
than those of more developed countries.

They used questionnaires to assess the personality traits of
parents in 65 families. They tested for the so-called "Big Five"
personality dimensions — extroversion, neuroticism, agreeableness, openness and
conscientiousness — traits considered the basic components to anyone's
personality.

Women with above average levels of neuroticism had 12
percent more children than those with below-average levels.

Men with above average extroversion (sociable and outgoing) had
a 31-percent higher chance of attaining a high social status than those with
below average extroversion. The extroverts were also 40 percent more likely to
have more than one wife, and had 14 percent more children than those with below-average
levels.

Neuroticism had a drawback when it came to the physical
fitness of the children. Young children (ages 0 to 5) of mothers with high
levels of neuroticism had a BMI that was 18 percent lower than children of
mothers with low neuroticism levels. Low BMI indicates the children could be
malnourished. However, this result was only true of families with low social
rank.

"You would expect that fewer of those children,
offspring of highly neurotic women, would survive in the long run," Jokela
said.

Indeed, neuroticism appeared to be a double-edged sword in
terms of reproduction, with the optimal amount being somewhere in the middle.
For women, an intermediate level of neuroticism was associated with the maximum
reproductive success, meaning having the most children that you would expect to
live to adulthood.

Many have suspected that personality provides some sort of
trade-off between the number of offspring and their health, but the current
study is one of the first to provide experimental evidence of such a trade-off,
Jokela said.

Why does neuroticism
mean more kids?

The scientists aren't sure why neuroticism leads to more
children in this society, but they have some speculations. Neuroticism has been
linked to competitiveness, and it's possible that, in a society where having
offspring is highly valued, the neurotic women have more babies to out-compete
other wives.

Also, neurotic women might be more sexually active than
others, which goes along with previous studies that have linked the personality
trait with attachment anxiety (think clingy partner) and high sexual motivation
in Westernized countries, the researchers say. In populations with less access
to birth control, a high sex drive
could translate into more children.

Interestingly, studies in developed countries have found
neuroticism actually decreases the probability of having large families, or has
no effect at all. This suggests the impact of a personality trait on fertility
can depend on the cultural context.

The researchers also don't know why children of neurotic
women were in poorer physical shape. But it could be that women with more
neuroticism have difficulties managing larger families, Jokela said.

The fact that the men's personality didn't impact the
children's physical shape could reflect the different roles men and women have
in child rearing in this society, Jokela said, with women having a larger role.

The results were published June 7 in the journal Proceedings
of the National Academy of Sciences.

Rachael has been with Live Science since 2010. She has a masters degree in journalism from New York University's Science, Health and Environmental Reporting Program. She also holds a Bachelor of Science in molecular biology and a Master of Science in biology from the University of California, San Diego.